1. Field of the Invention
The present invention relates to dielectric ceramic materials and monolithic ceramic capacitors, and in particular, to a dielectric ceramic material suitable for use in monolithic ceramic capacitors and monolithic ceramic capacitors including the dielectric ceramic material.
2. Description of the Related Art
A dielectric ceramic material of interest to the present invention is described in, for example, WO 2008/010412. The dielectric ceramic material described in WO 2008/010412 is mainly composed of a barium titanate-based complex oxide with a perovskite structure and contains secondary grains composed of an R—Ni—Ti—O-based crystalline complex oxide mainly containing a rare-earth element R, Ni, and Ti. The Ni may be partially substituted by Mg.
The foregoing dielectric ceramic material has high reliability such that problems do not occur even when a high field strength (e.g., about 20 kV/mm) is continuously applied to the dielectric ceramic material in a high temperature atmosphere for a long period of time. Thus, even when individual dielectric ceramic layers having smaller thickness are used to reduce the size and increase the capacity of a monolithic ceramic capacitor, the use of the dielectric ceramic material results in a high-reliability monolithic ceramic capacitor.
However, it was found that the foregoing dielectric ceramic material disadvantageously has a relatively low thermal shock resistance.
When a monolithic ceramic capacitor is mounted on a substrate by reflow soldering, the monolithic ceramic capacitor is in a heated condition for several minutes. Meanwhile, the melting point of solder has tended to increase with the trend toward lead-free solder. Thus, the monolithic ceramic capacitor is subjected to thermal shock, i.e., the monolithic ceramic capacitor is heated to as high as about 275° C. to about 325° C. during reflow soldering.
The thermal shock, i.e., a rapid temperature rise and rapid temperature drop, may cause cracks in the monolithic ceramic capacitor. Particularly in recent years, further progress toward a reduction in the thickness of layers to a very small thickness to less than about 1 μm, and to an increase in capacity, has increased the likelihood of cracks due to thermal shock.